NATURAL RADIATIONIntroduction

In the previous section, we discussed the deleterious effects that radiation can have on human beings (and indeed animals). This rather gives the impression that one should strive at all costs to avoid any sort of exposure to radiation! However, this is quite impossible, as radiation is all around us, and we are continually bombarded with radioactive particles.

The level of radiation exposure from natural sources occurs at the level of stochastic effects, discussed in the previous section, and the principal sources are as follows: 

1.                  Cosmic rays;

2.                  Naturally occurring radionuclides in soils and rock;

3.                  Naturally occurring radionuclides in our bodies;

4.                  Radon gas in our homes 

We will discuss each of these in turn. 

Cosmic Rays  

Cosmic rays are a source of radiation that originates from outer space. In fact there are two sources; the first is the sun, with the remainder originating from outside the solar system. Primary cosmic rays (those that are incident on the atmosphere of the earth) consist mainly of protons (hydrogen nuclei), and alpha particles (helium nuclei). Also present are electrons and a small number of heavier nuclei. 

The energy of the primary cosmic rays can be very high, and when these cosmic rays interact with the atoms and molecules in the atmosphere, a number of additional secondary particles are produced, including protons, electrons, neutrons, positrons and a variety of more ‘exotic’ particles that will not be discussed here. Many of these secondary cosmic rays are sufficiently energetic to reach the surface of the earth. 

At sea level, it has been found that the number of ions produced by cosmic rays is about 2 ions per cubic centimeter per second. Using this information, it can be calculated that the absorbed dose rate due to these cosmic rays is about 32 nGy/hr (0.000000032 Gy/hr). The world average annual dose rate due to cosmic rays is about 0.4 mSv/yr (0.0004 Sv/yr). 

It is of interest to note that the dose rate due to cosmic rays increases with altitude, because the flux of cosmic ray particles increases (they have less distance to travel through the earth’s atmosphere). Thus, regular travellers

by airplane, and indeed the pilots of those planes, will be subject to considerably higher radiation doses. The dose rate doubles for every 1500 m increase in altitude, and so at a cruising height of 10000 m, the dose rate could exceed 32 nGy/hr ´ 2(10/1.5) ~ 3.2 mGy/hr. This is 100 times greater than the dose rate at sea level. 

Supersonic aircraft often fly at much higher altitudes, perhaps as high as 15 km. The dose rate at this altitude could be as high as 210 ´ 32 nGy/hr = 33 mGy/hr. It is reassuring to note that supersonic aircraft carry monitoring equipment to measure the cosmic ray dose rate, so that during periods of intense cosmic radiation (for example during solar flares – the primary source of solar cosmic radiation), the pilot can adopt a lower flying height, resulting in a reduction of the radiation dose to the passengers and crew.

Radionuclides in Soils and Rock  

A number of radioactive materials occur naturally in the earth itself. These radioactive materials all have very long half lives, and have been present in the earth since its creation. At the creation of the earth during the Big Bang, perhaps about 15 billion years ago, virtually all possible atoms were created. Most of the radioactive atoms will have decayed away, but there are three important radionuclides that have such long half lives that even after 15 billion years, they are still present in the earth. These radionuclides are: 

Uranium-238 half life = 4.46 billion years;

Thorium-232 half life = 14 billion years

Potassium-40 half life = 1.3 billion years 

Other primordial radionuclides include rubidium-87 and uranium-235, but these are not so important in considering radiation doses to man. Uranium-235 is an important source of fuel for many types of nuclear reactor, and the extraction of concentrated amounts of uranium-235 from natural uranium in the ground is a major technological challenge. 

Uranium-238 and thorium-232 have the important property that their radioactive decay products are also radioactive. Both of these nuclides are at the beginning of long sequences of radionuclides (containing about 10 or 15 members) in which each member is radioactive, and decays to the next member in the series. Only the final member of the series consists of stable atoms. Uranium-235 is also at the head of a long sequence of radionuclides. 

The following diagram illustrates these two radionuclides and the radioactive progeny that arise from their decay.

 

  It follows that any of the members of the decay series mentioned above can give rise to a radiation dose to man. However, the nature of the radiation dose is usually considered in two categories, namely external doses, and doses that arise internally, for example because the nuclides have been inhaled or ingested. It turns out that the external doses from these natural nuclides are dominated by bismuth-214, which is a strong gamma emitter and occurs in the uranium-238 series, and thallium-208, which is also a gamma emitter and occurs in the thorium-232 series. The external dose, averaged over the world population, is about 0.5 mSv/yr. 

The internal doses arise when radioactive materials are ingested or inhaled. In this case, the nuclides that emit alpha particles assume a greater importance (for example radium-226 in the uranium-238 series). The dose arising from internal contamination (averaged over the world population) is about 0.3 mSv/yr. 

 

Radionuclides in the Body  

An important radionuclide that can be found in the human body is potassium-40 (it is also found in soils and rocks, and contributes to the doses considered in the previous section). The human body of a 70 kg man contains about 140 g of potassium, most of which is located in muscle. Of this potassium, about 0.018% is radioactive potassium-40. This corresponds to a mass of potassium-40 of about 0.03 g. It can be shown by calculation that the decay of this potassium-40 produces about 6000 radioactive decays every second, and therefore body tissues will acquire a radiation dose. The dose arising from potassium-40 in the body (and a contribution due to naturally occurring potassium-40 in rocks and soils) is about 0.1 mSv/yr. 

Radon in Homes  

One of the most important sources of natural radiation arises from an element called radon. The two most important isotopes of radon are radon-220 and radon-222. The former is often called thoron and the latter is usually given the name of the element, i.e. radon. To avoid confusion, we will reserve the term radon for the name of the element, and we will refer to each isotope explicitly. 

Radon has two important qualities from the perspective of radiation protection: 

1. Radon is a gas;

2. Both isotopes of radon are radioactive and are derived either from the decay of uranium-238 (radon-222) or thorium-232 (radon-220). 

Thus, because uranium-238 and thorium-232 occur to some extent in all soils and rocks, it follows that radon gas is constantly emanating from the ground. In areas that are particularly rich in uranium and/or thorium ores, the rate of emanation can be quite high. However, the radiological hazard from radon-220 (thoron) is usually smaller than that from radon-222, because radon-220 has a half-life of only 55 seconds, and so decays away before radiological problems can arise. Radon is colourless and odourless, so we cannot perceive the presence of radon in the atmosphere. However, in most circumstances, only radon-222 is problematic. 

Of particular importance is the fact radon can accumulate in enclosed structures such as homes and offices. The radon either seeps into buildings through the foundations, or in parts of the world where building materials are derived from local sources, the radon may enter the building as a result of emanation from the foundation and building materials themselves. If the degree of ventilation of the building is negligible, radon can accumulate to substantial levels. The radiological hazard is exacerbated by the fact that buildings such as homes tend to be occupied for substantial periods of time. 

Strictly speaking, radon-222 itself is not particularly dangerous, as radon is an inert gas, and so radon breathed in tends to be breathed straight back out again. However, the decay products of radon-222 are isotopes of bismuth and polonium, which are ‘heavy’ metals that attach themselves to dust particles, and tend to accumulate in the lung. These isotopes then undergo further radioactive decay, and bombard the lungs with alpha particles, resulting in radiological damage to the cells of the lung. Thus, lung cancer is a possible outcome from exposure to radon. It should be noted that cigarette smoking tends to increase the likelihood of lung cancer in regions where radon levels are high. 

The hazard of radon is taken so seriously that action levels are defined in most countries. These action levels specify the maximum concentration of radon and its decay products that is allowed before intervention is required to lower concentrations. The most common type of intervention is the use of fans under the floor of an affected building to ‘suck’ radon out of the underfloor region, before it can enter the main volume of the building.  

 

Ionising Radiation - The End  

At this point, it was my intention to describe some of the more practical uses of radiation. However, it became clear to me that (1) there are so many uses that I could not do justice to them all; (2) I may never actually finish writing this article if I started out to do that, and; (3) people’s views differ on what are acceptable, unacceptable or controversial uses. 

There are a multitude of stories (both happy and sad) about how people’s lives have been changed as a result of radiation. But that’s for another time!